Question 1

What is dynamic viscosity?

Accepted Answer

Dynamic viscosity (also called absolute viscosity) measures a fluid's resistance to shear deformation. It is defined as the shear stress divided by the shear rate (velocity gradient). The SI unit is Pascal-second (Pa·s). In simple terms: highly viscous fluids like honey resist flow much more than low-viscosity fluids like water.

Question 2

What is the difference between Poise and centipoise?

Accepted Answer

The Poise (P) is the CGS unit of dynamic viscosity: 1 P = 0.1 Pa·s. The Centipoise (cP) = 0.01 P = 0.001 Pa·s = 1 mPa·s. Water at 20°C ≈ 1.002 cP, making cP a convenient unit because common liquids fall in the 1–100,000 cP range. cP = mPa·s numerically, so they are interchangeable.

Question 3

What are typical viscosity values?

Accepted Answer

Air at 20°C: 0.0181 cP. Water at 20°C: 1.002 cP. Blood: 3–4 cP. Engine oil SAE 10W: ~100 cP at 20°C. Engine oil SAE 30: ~110 cP at 20°C. Honey: 2,000–10,000 cP. Ketchup: 50,000–70,000 cP. Glass: ~10²³ cP (effectively a very slow-flowing solid).

Question 4

What is lbf·s/ft² (pound-force-second per square foot)?

Accepted Answer

This is the Imperial/US unit of dynamic viscosity. It equals 47.88 Pa·s. It is sometimes called the 'Reyn' in some older textbooks (after the fluid unit system). 1 slug/(ft·s) = 1 lbf·s/ft². It is used in some US engineering applications, particularly in aerospace and fluid mechanics calculations using the British Gravitational system.

Question 5

How do I convert between dynamic and kinematic viscosity?

Accepted Answer

Kinematic viscosity (ν) = Dynamic viscosity (μ) ÷ Density (ρ). For water at 20°C: ν = 1.002 cP ÷ 0.9982 g/cm³ ≈ 1.004 cSt. For engine oil SAE 30 at 20°C: ν ≈ 110 cP ÷ 0.875 g/cm³ ≈ 126 cSt. You need the fluid density to make this conversion.

Viscosity – Dynamic Converter

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